Combination converters with a first secondary winding, which is wound onto a non-magnetic annular core, and a second secondary winding, which is wound on a magnetic annular core (iron core), are known. In this case, both secondary windings are arranged in a common housing which has a pot shape with a hollow passage cylinder integrally formed on the housing floor. In this case, the first secondary winding serves to measure current (converter for measuring current) and the second secondary winding serves to supply current (converter for supplying current), wherein one current conductor is routed through the hollow passage cylinder and the annular cores and forms the primary winding of the converters. The magnetic annular core is preferably composed of soft iron.
Current converters for circuit breakers have to have a high dielectric strength, that is to say correspondingly long air and creepage paths. These are required, in particular, in order to withstand the so-called surge or EMC testing.
In order to achieve a high dielectric strength, it is known, in principle, to embed the two secondary windings into an encapsulation compound and to route the connection wires of the windings through the encapsulation compound to the outside. The encapsulation compound in the form of insulation means has to be appropriately certified for industrial use.
Nowadays, particularly in the case of current converters for circuit breakers, it is often necessary for said current converters to be free of silicone in order to avoid the precipitation (evaporation) phenomena which occur with silicone under certain conditions.
Therefore, possible encapsulation compounds often include only resins, in particular resins which are composed of two components, that is to say epoxy resins. However, said resins have the disadvantage that there is a loss of volume when the encapsulation compound is chemically cross-linked, this being associated with pressure on the windings which reduces, in particular, the permeability of the iron core when said iron core is composed of soft iron.
Current converters for circuit breakers further need to be designed for a large operating temperature range (for example of −25° C. to approximately 180° C.). It is sometimes necessary to ensure a storage temperature of up to −40° C. In fact, cracks occur in the encapsulation compound specifically at relatively high temperatures around 180° C. in the case of resins, said cracks, in turn, forming undesired creepage paths.